1. Field of the Invention
The present invention relates to a nonvolatile memory and an erasing method of erasing information written in the nonvolatile memory.
2. Description of the Related Art
Each memory cell in semiconductor memories such as a DRAM (Dynamic Random Access Memory) or an EEPROM (Electrically Erasable Programmable Read-Only Memory) is usually composed of a transfer element such as a transistor and a storage element such as capacitor or a transistor with a floating gate. Semiconductor memories with high storage capacity have been conventionally realized by an advance of a fine processing technique, and it is expected that finer transfer elements would be realized in the future. On the other hand, realization of finer storage elements has been already up against a difficulty. This obstructs realization of semiconductor memories with higher storage capacity.
As a memory capable of realizing a higher storage capacity, the present inventors propose a nonvolatile memory using photochromic molecules for a storage element in Applied Physics Letters, Volume 83, Number 5, Pages 937-939.
The photochromic molecule used in the memory is a bistable molecule which can take two stable structures in a regular operating temperature range. Isomerization of the bistable molecule from a first isomer to a second isomer and its reverse reaction can be brought about when irradiated with light rays having different wavelengths, i.e., write light and erase light, respectively.
The isomerization from the first isomer to the second isomer can also be brought about by injecting holes and electrons into the bistable molecular layer containing the bistable molecules. That is, information can be electrically written in the nonvolatile memory. Further, since the electroconductive characteristics of the storage element vary in accordance with a molar ratio of the first isomer to the second isomer, information written in the nonvolatile memory can be electrically read out.
However, in order to erase information written in the nonvolatile memory, the bistable molecular layer must be irradiated with the erase light. The erase light irradiation is carried out to a relatively large area, which is typically an area corresponding to all memory cells, unless a mechanical control is performed. Since the mechanical control considerably lowers an erasing speed, it is practically not possible to selectively erase a part of the information written in this nonvolatile memory.